The invention relates to a process for the production of 2,2xe2x80x2-dithiazolyl disulfides by oxidation of 2-mercaptothiazoles with peroxidic compounds in an aqueous suspension in a specific pH range.
In the industrial production of dibenzothiazyl disulfides by oxidation of 2-mercaptobenzothiazoles, many different oxidizing agents have already been used (Ullmanns Encyclopedia of Industrial Chemistry, 5th ed. vol. A-26, p. 773-8, VCH, Weinheim, Basel, Cambridge, New York, Tokyo, 1995). Thus, oxidation with sodium chlorate and sodium nitrite solution in a hydrochloric acid medium at 30xc2x0 C. is prior art. However, this process has a series of disadvantages. The consumption of mineral acid is very high (3 moles of HCl per mole of 2-mercaptobenzothiazole) and large quantities of co-products are formed. It is also known to perform the oxidation of 2-mercaptobenzothiazoles using nitrous acid. According to the process of U.S. Pat. No. 1,908,935, 2-mercaptobenzothiazole is suspended in water, a water-soluble nitrite is added and oxygen or an oxygen-containing gas, such as air, is passed through the reaction mixture. At the same time a mineral acid, which releases nitrous acid from the nitrite, is added. In the process according to U.S. Pat. No. 2,119,131 and U.S. Pat. No. 3,062,825, stoichiometric quantities of nitrite are used as the sole oxidizing agent. As a result, a more rapid and more complete reaction is achieved. These oxidation processes are also disadvantageous in so far as the consumption of mineral acid is again very high here, and salts and nitrogen oxides are formed in large quantities as by-products.
Chlorine has also been used as an oxidizing agent (Kirk-Othmer, Encyclopedia of Polymer Science and Technology (1970), vol. 12, p. 262). However, this is a complicated reaction with critical reaction conditions in which large quantities of superoxidized by-products are often formed. According to DE-A 23 09 584, to increase the product yield and reduce the quantity of excess chlorine required for adequate oxidation, separate streams of an aqueous solution of an alkali metal salt of mercaptobenzothiazole, an aqueous solution of an alkali metal hydroxide and gaseous chlorine are continuously reacted with one another under the surface of the liquid, with vigorous stirring, at 20 to 75xc2x0 C., the pH and the redox potential of the aqueous mixture being kept at pH 7 to 10 and a redox potential of xe2x88x92150 to 250 mV by regulating the feed of the aqueous hydroxide solution and the gaseous chlorine. This process also requires very careful control in order to prevent the further oxidation of dibenzothiazyl disulfide to benzothiazyl-2-sulfinate and -sulfonate. The process is also disadvantageous because large quantities of alkali hydroxide are consumed and large quantities of common salt are formed as a co-product.
Hydroperoxides, such as hydrogen peroxide, alkyl hydroperoxides and aralkyl hydroperoxides, have also already been used as oxidizing agents in the production of dibenzothiazyl disulfide (cf. e.g. DE-A 23 49 314). However, the use of a low aliphatic alcohol as solvent is expressly required here. The use of organic solvents is disadvantageous for an industrial process, however, since they can represent both an environmental hazard and, owing to their high inflammability, a constant fire hazard. In addition, organic solvents have to be recycled, purified and disposed of after use, which is expensive.
The oxidation of heterocyclic thiols to disulfides with the aid of hydrogen peroxide or organic peracids in water or organic solvents or in mixtures thereof is described in EP-A 194 571 A1. However, no specific pH which must imperatively be adhered to during these reactions is disclosed therein. In the examples there is only a single reaction in an aqueous medium, in which, however, the thiol is reacted in great dilution (2% solution) and in a homogeneously dissolved form, with hydrogen peroxide. In this way, which is described herein, a very large quantity of water is required in the reaction and a correspondingly large quantity of wastewater is produced, which must be disposed of at great cost. No possibility of working heterogeneously in concentrated suspension, thus saving solvent or wastewater, is disclosed.
This possibility of reacting 2-mercaptothiazoles with hydroperoxides, especially with hydrogen peroxide, in concentrated aqueous suspension is described in U.S. Pat. No. 4,463,178. According to this publication, however, the use of an aqueous amine solution, such as e.g. ammonia or alkylamine solution, is explicitly required as a solubility promoter for the otherwise water-insoluble 2-mercaptothiazole. It is not disclosed that the oxidation reaction can take place without any problems and with quantitative yields even without any solubility-promoting auxiliary agent in pure water.
EP-A 008 548 describes the use of hydrogen peroxide as an oxidizing agent in combination with ethylenediaminetetraacetic acid or the salts thereof. The reaction times have to be very long here, over 24 hours in some cases, to achieve complete conversion. This is a great disadvantage for a large-scale process.
Common to all the above-mentioned oxidation processes is the disadvantage that comparatively expensive oxidizing agents, together with acids, bases, solvents or other auxiliary substances, are required and, in some cases, unusable co- or by-products are also formed.
A process for the electrolytic oxidation of 2-mercaptobenzothiazole to dibenzothiazyl disulfide should also be mentioned (cf. DE-A 27 43 629). This process is technically complex and therefore less economical.
There is still, therefore, a need to create an improved process for the oxidation of 2-mercaptothiazoles by means of peroxidic compounds.
Therefore, the present invention provides a process for the production of 2,2xe2x80x2-dithiazolyl disulfides of the general formula 
wherein
R and R1 can be the same or different and each denote hydrogen, halogen, nitro, hydroxyl or optionally substituted C1-C12 alkyl or alkoxyl or C6-C12 cycloalkyl or aryl or C1-C12 heteroaryl, or jointly form the residue 
wherein
R2 to R5 have the same meaning as R and R1, by oxidation of a corresponding substituted 2-mercaptothiazole with peroxidic compounds, characterized in that the oxidation is performed in an aqueous suspension at a pH in the range of 6.5 to 8.0.
The present invention relates to a process for the production of 2,2xe2x80x2-dithiazolyl disulfides of the general formula 
wherein
R and R1 can be the same or different and each denote hydrogen, halogen, nitro, hydroxyl or optionally substituted C1-C12 alkyl or alkoxyl or C6-C12 cycloalkyl or aryl or C1-C12 heteroaryl, or jointly form the residue 
wherein
R2 to R5 have the same meaning as R and R1, by oxidation of a corresponding substituted 2-mercaptothiazole with peroxidic compounds, characterized in that the oxidation is performed in an aqueous suspension at a pH in the range of 6.5 to 8.0, preferably 6.8 to 7.5.
In the above formula, fluorine, chlorine, bromine or iodine, preferably chlorine or bromine, are suitable as halogen residues.
C1-C12 alkyl is understood to mean all linear or branched alkyl residues with 1 to 12 C atoms known to the person skilled in the art, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, n-pentyl, i-pentyl, neo-pentyl and hexyl, which, for their part, can, in turn, be substituted. Suitable substituents in this case are halogen, nitro, hydroxyl, or else C1-C12 alkyl or alkoxy, and C6-C12 cycloalkyl or aryl, such as benzoyl, trimethyl phenyl, ethyl phenyl, chloromethyl, chloroethyl and nitromethyl.
C1-C12 alkoxyl is understood to mean all linear or branched alkoxyl residues with 1 to 12 C atoms known to the person skilled in the art, such as methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, t-butoxy, n-pentoxy, i-pentoxy, neo-pentoxy and hexoxy, which, for their part, can, in turn, be substituted. Suitable substituents in this case are halogen, nitro, hydroxyl or else C1-C12 alkyl or alkoxyl, and C6-C12 cycloalkyl or aryl.
C6-C12 cycloalkyl is understood to mean all mono- or polynuclear cycloalkyl residues with 6 to 12 C atoms known to the person skilled in the art, such as cyclohexyl, cycloheptyl, cyclooctyl and cyclononyl, which, for their part, can, in turn, be substituted. Suitable substituents in this case are halogen, nitro, hydroxyl or else C1-C12 alkyl or alkoxyl, and C6-C12 cycloalkyl or aryl, such as methylcyclohexyl, chlorocyclohexyl and nitro-cyclohexyl.
C6-C12 aryl is understood to mean all mono- or polynuclear aryl residues with 6 to 12 C atoms known to the person skilled in the art, such as phenyl or naphthyl, which, for their part, can, in turn, be substituted. Suitable substituents in this case are halogen, nitro, hydroxyl or else C1-C12 alkyl or alkoxyl, and C6-C12 cycloalkyl or aryl, such as bromophenyl, chlorophenyl, toluyl and nitrophenyl.
C1-C12 heteroaryl is understood to mean all mono- or polynuclear heteroaryl residues known to the person skilled in the art which, in addition to 1 to 12 C atoms, also contain one or more heteroatoms, such as N, S, O and/or P, in the aromatic ring system, such as pyridinyl, triazinyl, furyl, thienyl, thiazolyl, thiazinyl, pyrrolyl, quinolinyl which, for their part, can, in turn, be substituted by the above-mentioned substituents.
The residues R-R5 in the formula preferably denote hydrogen, methyl, ethyl, propyl, t-butyl, methoxy, ethoxy, cyclohexyl, benzoyl, methoxy, ethoxy, phenyl, naphthyl, chlorophenyl, toluyl and nitrophenyl.
2,2xe2x80x2-Dithiazolyl disulfides are used e.g. as vulcanizing agents for rubber. The process according to the present invention is particularly significant for the production of 2,2xe2x80x2-dibenzothiazolyl disulfide, a most preferred embodiment of this class of compounds. However, it is also suitable and successful in the production of other compounds of this type. For the preferred production of 2,2xe2x80x2-dibenzothiazolyl disulfide (MBTS), 2-mercaptobenzothiazole (MBT) is used as the starting substance. Examples of other 2-mercaptothiazoles that are suitable as starting substances for the production of other 2,2xe2x80x2-dithiazolyl disulfides of the general formula (I) include the compounds mentioned in DE-A 23 55 897, such as
2-mercaptothiazole
2-mercapto-4-methylthiazole
2-mercapto-4-ethylthiazole
2-mercapto-4-n-propylthiazole
2-mercapto-4-n-butylthiazole
2-mercapto-4,5-dimethylthiazole
2-mercapto-4,5-di-n-butylthiazole
2-mercapto-4-phenylthiazole
2-mercapto-5-chloro-4-phenylthiazole
2-mercapto-4-p-bromophenylthiazole
2-mercapto-4-m-nitrophenylthiazole
2-mercapto-4-m-chlorophenylthiazole
2-mercapto-4-methylbenzothiazole
2-mercapto-5-methylbenzothiazole
2-mercapto-6-methylbenzothiazole
2-mercapto-4,5-dimethylbenzothiazole
2-mercapto-4-phenylbenzothiazole
2-mercapto-4-methoxybenzothiazole
2-mercapto-6-methoxybenzothiazole
2-mercapto-5,6-dimethoxybenzothiazole
2-mercapto-6-methoxy-4-nitrobenzothiazole
2-mercapto-6-ethoxybenzothiazole
2-mercapto-4-chlorobenzothiazole
2-mercapto-5-chlorobenzothiazole
2-mercapto-6-chlorobenzothiazole
2-mercapto-7-chlorobenzothiazole
2-mercapto-5-chloro-6-methoxybenzothiazole
2-mercapto-5-chloro-4-nitrobenzothiazole
2-mercapto-5-chloro-6-nitrobenzothiazole
2-mercapto-4,5-dichlorobenzothiazole
2-mercapto-4,7-dichlorobenzothiazole
2-mercapto-5-nitrobenzothiazole
2-mercapto-6-nitrobenzothiazole
2-mercapto-4-phenylbenzothiazole
2-mercapto-naphthothiazole
2-mercapto-6-hydroxybenzothiazole.
As mentioned, peroxidic compounds, especially hydrogen peroxide, alkyl hydroperoxides or aralkyl hydroperoxides, are used as oxidizing agents. Naturally, mixtures of these can also be used. All peroxides described in DE-A-2 349 314 can be used as alkyl hydroperoxides and aralkyl hydroperoxides. However, hydrogen peroxide is preferred. In general, the peroxide concentration used in the process according to the present invention is in the range of 3 to 50 wt %. For economic reasons, peroxide concentrations of 5 to 35 wt % are preferably used, more preferably, 10 to 35 wt %.
Water is used as the solvent for the process according to the present invention. However, water-miscible organic solvents that are stable to oxidation can also be added to the water. Examples of these are alcohols and ketones, dimethylformamide and acetone, and mixtures thereof. Suitable alcohols are e.g. aliphatic alcohols with 1 to 10 carbon atoms, especially methanol, ethanol, propanol, isopropanol, n-butanol, sec.-butanol, tert.-butanol, pentanol, hexanol, heptanol and octanol. The concentration of the solvent in the water is not critical. In general, the quantity of solvent is in the range of 1 to 10 wt %, based on the quantity of water. Larger quantities of solvent should be avoided for economic reasons, since in these cases, larger quantities of solvent also have to be processed or disposed of.
Naturally, it is also possible to produce or release the peroxides, especially hydrogen peroxide, in situ from suitable precursors.
The pH of the process according to the present invention must imperatively lie within the range stated from beginning to end of the reaction. If it falls below this range, the yield and purity of the end product are reduced. If the pH range is exceeded, the yield and selectivity of the reaction are reduced and by-products are formed which pass into the end product and also into the wastewater. As a result, the product according to the present invention is contaminated and the wastewater polluted with organic material.
In order to prevent pH fluctuations during the reaction and to keep the reaction precisely within the pH range specified, an efficient measurement and control technique can be used, consisting e.g. of an in situ pH measurement and an electronically controlled metering system for the acid or base to be added as required.
Suitable acids and bases are known to the person skilled in the art; their selection is not critical. Acids preferably used are e.g. sulfuric, hydrochloric or phosphoric acid. Preferred bases are e.g. aqueous solutions of ammonium hydroxide or of basic metal hydroxides from groups 1 to 13 of the periodic table, preferably sodium hydroxide or potassium hydroxide.
In order to be able to compensate very rapidly, even for smaller pH fluctuations, a buffer system is preferably used, adapted to the required pH range. Suitable buffers for this purpose are known to the person skilled in the art and can be found e.g. in xe2x80x9cRxc3x6mpp Chemie Lexikonxe2x80x9d, Thieme Verlag Stuttgart, 9th edition, vol. 5, (1992) 3677 or in the xe2x80x9cCRC Handbook of Chemistry and Physicsxe2x80x9d 79th ed. (1998) 8-43.
For example, buffers can be used which are composed of a mixture of metal hydrogen carbonates and metal carbonates, or of a mixture of metal hydrogen phosphate and metal phosphate, the metals coming from groups 1 to 13 of the periodic table, and e.g. of a mixture of nitrogen-containing bases, such as triethanolamine, tris(hydroxymethyl)aminomethane or imidazole and their ammonium salts, formed by reaction with an acid, e.g. hydrochloric acid or sulfuric acid. The components of the buffer, e.g. the metal salts mentioned, can be used individually or mixed together.
Buffers composed of a mixture of sodium or potassium salts with anions of hydrogen carbonate and carbonate, or of dihydrogen phosphate, hydrogen phosphate and phosphate, or of tris(hydroxymethyl)aminomethane and its ammonium salts, are preferably used.
The buffer can be used both as the only pH control and, preferably, in combination with an efficient regulating system as described above.
The quantity of buffer used and the corresponding buffer capacity depend on the strength of the pH fluctuation to be expected, which is, in turn, dependent on the reaction conditions, such as temperature and rate of metering and the efficiency of the pH regulating system used, and can readily be determined by appropriate preliminary tests.
The reaction temperature in the process according to the present invention is about 0 to 150xc2x0 C., preferably 20 to 90xc2x0 C. and more preferably 30 to 70xc2x0 C. At lower temperatures, the rate of reaction decreases, and at higher temperatures the selectivity of the reaction is reduced.
The reaction period is generally 0.5 to 10 hours under the above reaction conditions.
The process according to the present invention is performed in a simple manner, e.g. in that the 2-mercaptothiazole is dispersed in the reaction medium in powdered form, a buffer is optionally added, dissolved or in solid form, and the hydrogen peroxide is allowed to flow in under the pressure and temperature conditions stated, preferably with stirring.
In the process according to the present invention, practically quantitative yields and selectivities of more than 98% are achieved. The 2,2-dithiazolyl disulfides obtainable are distinguished by high purity and can be used directly as rubber vulcanizing agents, for example, without further purification.
The 2,2xe2x80x2-dithiazolyl disulfides that can be produced according to the present invention are excellently suited as vulcanization accelerators in sulfur-containing rubber mixtures. Dibenzothiazyl disulfide is especially suitable.